I got a question from a very eloquent adult writer who wanted to know how to keep her many characters straight as she wrote her book. My answer: this is a monstrous problem, especially if you're writing a series. I often copy ads off the Internet that look somewhat like the character they represent. For Shadow Souls, I even carefully picked out real clothes from expensive prom offerings so that I could visualize the dresses on the girls more clearly. I may post them on my website. Of course the girls aren’t perfect in representing my characters, but some of the dresses were!
The traditonal method used to have you make up an index card for each character and write a physical, mental, emotional, spiritual etc. description for them on the index card. I tend to do my work on the computer--it doesn't get lost or fade over the years. I write for each book or series a list of characters and a table that shows what color eyes and hair they have (so that I don't end up with two unrelated mink-brown-haired violet eyed girls). I do a mini character bible for new characters (like the index card bios). I already have bios for my main characters that tell me where they live and what their bedroom--or wherever they tend to hang out--looks like, and the like. Sometimes I do maps. I tend to be longwinded so my entry for Elena Gilbert is about five single spaced pages long.
And nothing keeps me from making huge bloopers. Just as I'm writing you now, at 5 a.m. my time, I tend to do a LOT of writing each day. I get confused and sometimes use my memory rather than my Guide, which is like using a sieve instead or a pot to cook with. The copy-editor is supposed to catch such mistakes if the editor misses them, but I know of at least two that they didn’t in the last couple of books. One was out and out my fault, but never caught—that was referring to Tyron in Nightfall as Dr. Alpert’s grandson, and then in Shadow Souls as her son. Solution: I gave Tyrone a divorced mom who died young, so that his grandma adopted him. Not a very good fix but the best I could pull off by the time I was writing Midnight.
There is also software out, like New Novelist (which I haven't tried) that is built so you can write in character descriptions and traits and leaf through them whenever you like. It also claims to help you write. I'd take that with a grain of salt.
If you can draw, the index card method may work best for you. You could just flip through images until you got the right one. Otherwise, I'd recommend a computer . . . and a lot of rather tedious typing. But you can also gain new insights into your characters if you do a Guide entry on them--they open up to you and often show you that they have hobbies or habits that you never knew about before. Matt rattles his change in his pocket when disturbed. Stefan pinches the bridge of his nose. Damon just smiles like an archangel.